Direct-Current (DC) Cardioversion-Defibrillation
The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic drugs, direct current (DC) cardioversion-defibrillation, implantable cardioverter-defibrillators (ICDs), pacemakers (and a special form of pacing, cardiac resynchronization therapy), catheter ablation, surgery, or a combination, is used.
A transthoracic DC shock of sufficient magnitude depolarizes the entire myocardium, rendering the entire heart momentarily refractory to repeat depolarization. Thereafter, the most rapid intrinsic pacemaker, usually the sinoatrial (SA) node, reassumes control of heart rhythm. Thus, DC cardioversion-defibrillation very effectively terminates tachyarrhythmias that result from reentry. However, it is less effective for terminating tachyarrhythmias that result from automaticity because the return rhythm is likely to be the automatic tachyarrhythmia. For tachyarrhythmias other than ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT), the DC shock must be synchronized to the QRS complex (called DC cardioversion) because a shock that falls during the vulnerable period (near the peak of the T wave) can induce VF. In VF, synchronization of a shock to the QRS complex is neither necessary nor possible. A DC shock applied without synchronization to a QRS complex is DC defibrillation.
Direct current (DC) cardioversion or defibrillation can be delivered as monophasic or biphasic current. Monophasic current travels in one direction between the two electrodes. In biphasic devices, the current reverses direction part way through the shock waveform. The biphasic device requires lower energy and has been shown to lead to higher rates of return of spontaneous circulation (ROSC). However, survival outcomes are similar in both devices (1). Most manual and automated external defibrillators (AEDs) are now biphasic because of increased efficiency at restoring sinus rhythm. Biphasic devices are also smaller in size (making the devices more portable).
When DC cardioversion is elective, patients should fast for 6 to 8 hours to avoid the possibility of aspiration. Because the procedure is frightening and painful, brief general anesthesia or IV analgesia and sedation (eg, fentanyl 1 mcg/kg, then midazolam 1 to 2 mg every 2 minutes to a maximum of 5 mg) is necessary. Equipment and personnel to maintain the airways must be present.
The electrodes (pads or paddles) used for cardioversion may be placed anteroposteriorly (along the left sternal border over the 3rd and 4th intercostal spaces and in the left infrascapular region) or anterolaterally (between the clavicle and the 2nd intercostal space along the right sternal border and over the 5th and 6th intercostal spaces at the apex of the heart). After synchronization to the QRS complex is confirmed on the monitor, a shock is given.
The most appropriate energy level varies with the tachyarrhythmia being treated. Cardioversion and defibrillation efficacy increases with use of biphasic shocks, in which the current polarity is reversed part way through the shock waveform.
For defibrillation of ventricular fibrillation or pulseless ventricular tachycardia the energy level for the first shock is
Subsequent shocks are at the same or higher energy level for biphasic devices and are at the same level for monophasic devices.
For synchronized cardioversion of atrial fibrillation the energy level for the first shock is
Subsequent shocks are at the same or higher energy level for both biphasic and monophasic devices.
DC cardioversion-defibrillation can also be applied directly to the heart during a thoracotomy or through use of an intracardiac electrode catheter; then, much lower energy levels are required.
1. Schneider T, Martens PR, Paschen H, et al: Multicenter, randomized, controlled trial of 150-J biphasic shocks compared with 200-J to 360-J monophasic shocks in the resuscitation of out-of-hospital cardiac arrest victims. Circulation 2000 102:1780–1787, 2000.